There has been a paradigm shift in the oil and gas industry with the advent of drilling wells having a horizontal leg and then conducting multiple frac jobs along the horizontal leg. This has allowed the development of oil and gas bearing formations which were heretofore uneconomic. For some years, the fastest growing segment of hydrocarbon production in the United States has been from shales or very silty zones that previously have not been economic. The current areas of increasing activity include the Barnett Shale, the Haynesville Shale, the Fayetteville Shale, the Marcellus Shale, the Eagle Ford Shale and other shale or shaley formations. It is no exaggeration to say that the future of oil and gas production in the United States involves drilling horizontal wells and then conducting multiple frac jobs in the horizontal leg of the well. Any development that improves the efficiency or lowers the cost of drilling and completing multi-fraced horizontal wells is important to the production of oil and gas.
One standard technique for completing a horizontal well is to run a string of casing into the horizontal leg of the well and then cement the casing in place by pumping a first cementing plug through the casing followed by cement followed by a second cementing plug. Drilling mud in the annulus between the well bore and the casing is displaced and flows toward the surface and ultimately discharges into surface equipment. The first cementing plug ultimately lands in float equipment near the end of the casing string whereupon cement flows out of the casing, through passages in the float equipment and then flows in the annulus toward the surface. When the second cementing plug reaches the float equipment, it latches in place and prevents back flow of cement into the casing. Thus, at the end of a successful cement job, a sheath of cement sets up around the casing string from the float shoe or toe of the casing string toward or beyond the heel where the well bore turns horizontal.
Although there are a variety of ways to conduct frac jobs on a well, one standard technique is to perforate the horizontal well near the float shoe by running a perforating gun on coiled tubing to a location adjacent the float shoe, perforate the casing, pull the coiled tubing out of the well and then release the coiled tubing unit. This is called perforating the toe because the end of the casing string is sometimes referred to as the toe.
The next step in the standard technique is to pump into the well a perforating gun having a rubber sealing against the inside of the casing and shoot a set of frac perforations near the end of the casing string. It will be seen that it is necessary to perforate the toe because, otherwise, the inside of the casing string is pressure tight and the perforating equipment cannot be pumped into the well because there is no place for the liquid to escape that is in front of the perforating equipment. Thus, a coiled tubing run is necessary to transport a perforating gun to the toe so a set of frac perforations can be shot.
A first zone is fraced by pumping a slurry of water, chemicals and proppant through the casing and through the perforations into the hydrocarbon bearing formation. To conduct multiple frac jobs, the casing is perforated in a second location nearer the surface, a bridge plug is placed to isolate the zone that has been fraced and a second frac job is done by pumping frac fluids through the second set of perforations. This process is repeated until the desired number of fracs have been done.
It has been proposed to provide a system where the second cementing plug moves a sleeve and opens a port through the float shoe in the process of cementing a horizontal well. This theoretically provides a passage through the casing toe which allows a wire line perforating gun to be pumped into the well so a coiled tubing run is not required. One problem with this system is the cement has not set up when the port is opened, meaning that cement is often found inside the casing above the float equipment. This is a problem because the only known means of removing set up cement inside the casing string is with a bit on the end of a work string, which is usually a coiled tubing unit, meaning that the intended purpose of avoiding a coiled tubing run has been thwarted. In addition, it is not a good idea to remove cement from outside the casing, where it is needed, and let it flow into the casing where it is a problem.
Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 2,707,997; 4,673,039; 5,103,911; 5,425,424; 6,047,773; 6,397,950; 7,640,988 and 7,802,627, U.S. Printed Patent Applications 2006/0207763 and 2011/0017453, Canada Patent 2,692,377 and Great Britain Patent 2,452,858.